1. Technical Field
The present invention relates to injection devices for the dispensing of liquids, and more particularly to automatic injection devices (so called “auto-injectors”).
2. Technical Background
Automatic injection devices are typically configured such that when activated by a user, e.g. by pressing a button etc., an injection is automatically delivered. In order to automatically deliver an injection, such devices may be configured to automatically advance the needle of a syringe for insertion into the body of a user, depress the plunger to dispense liquid from the syringe barrel, and retract the needle. It will be appreciated that automatic injection devices in accordance with the present invention may be used in conjunction with veterinary applications, as well as applications to the human body, and thus references herein to the “body”, or such like, may refer to the human or animal body.
Certain issues need to be considered when designing an automatic injection device. Syringes are subject to strict regulations. In order to gain acceptance, an injection device should be compatible with existing syringes which have already been granted regulatory approval. This avoids the need to seek further regulatory approval which would otherwise be required if any modification of the syringe were required to cooperate with the remainder of the device. The injection device must also comply with ever increasing regulations aimed at reducing risk of needle stick injuries.
Further considerations are involved in avoiding breakage or damage to the syringe incorporated in the device during actuation. It is often necessary to drive the plunger of the syringe forward with considerable force to ensure that an intended dose is dispensed, and to force it through the tissues of the body to the desired delivery depth, particularly when the liquid to be dispensed is relatively viscous, or if it is to be injected to a greater depth. Forces exerted on the plunger may be transmitted to the syringe body via the relatively incompressible liquid disposed in the syringe, and may result in breakage or damage to the syringe, particularly to the syringe barrel, which is often formed of glass.
One automatic injection device is disclosed in GB 2410188. The Applicant has realised that this arrangement suffers from certain disadvantages. For example, this arrangement relies upon the radial tags of an inner housing engaging with the plunger and barrel, or plunger only, or, to transmit a driving force to the plunger and barrel, or plunger at the appropriate times during the actuation cycle. This causes axial movement of the plunger and barrel, or plunger relative to the outer housing during the needle advancing and dispensing stages respectively. These radial tags also engage the inner wall of the outer housing. As the inner housing moves axially within the outer housing, the tags scrape along the inner wall of the outer housing, creating friction opposing the axial movement of the plunger and/or barrel. This may in turn mean that a greater force must be exerted to drive the inner housing forward and overcome the frictional forces generated, especially when the liquid to be dispensed is a more viscous liquid. Further frictional forces may arise in these arrangements in the axial direction as the radial tags slide out of contact with the plunger and barrel.
Another problem which the Applicant has identified may arise with the arrangement of GB 2410188 relates to the way in which the radial tags of the inner housing engage the rear flange of the syringe barrel to drive it forward. This rear flange is often referred to as the “finger flange” because, when a manual injection is delivered using a syringe, the user's fingers rest in front of the flange to provide the necessary resistance to allow depression of the plunger using the thumb. The rear flange is a particularly delicate part of the syringe, which is often subject to manufacturing flaws, and is therefore susceptible to breakage. By driving the barrel forward by direct contact between the radial tags and the rear flange, a significant risk of breakage may arise as a result of the large forces which may be required to drive the barrel forward into the skin, and overcome the frictional resistance to the movement of the radial tags, especially for deeper injections.
In practice, this problem is compounded, as the forward radial tags may not abut the rear flange prior to activation of the device. Some clearance between the finger flange and the tags prior to activation may be present to introduce tolerance with respect to the axial position of the syringe relative to the inner and outer housing, and ensure that the flange will abut the radial tags once the inner housing moves forward. Variation in the axial position of the syringe with respect to the inner and outer housing in devices of the type shown in GB2410188 may be introduced during manufacture, or for example, if the device is dropped or held in certain orientations, or due to variations in atmospheric pressure or the way in which the device is filled with liquid prior to use. Variation may also arise if the device is used in conjunction with different sizes of syringe. It is then necessary to design the device to accommodate the largest size of syringe which may be used, such that a significant gap may exist between the radial tags and flanges of a syringe unless it is at the upper end of the possible size range. Such variation in the axial position of the syringe relative to the housing may cause variation of the dose delivered, or in extreme cases, the radial tags may even come to rest forward of the finger flange resulting in failure of the device to operate. Clearance between the finger flange and the tags may therefore be introduced in an attempt to accommodate any such variation. For example, GB 2447339 discloses modifications of the arrangements in GB2410188 including means to bias the barrel forward of the tags which act on the finger flange to try to ensure that the tags will engage the finger flange once driven forward. The Applicant has realised that when the radial tags must be driven forward through the air in a gap between the tags and the flange to engage the finger flange in this way, a greater driving force must be used to be sufficient to overcome the inertia of the finger flange and cause it to start moving with the inner housing. The stationary syringe finger flange will experience significant forces upon impact with the radial tags, which will already be moving forward axially at speed, increasing further the risk of damage to the flange and/or syringe barrel.